This invention pertains to the oxidation of aliphatic hydrocarbons, such as alkanes and monoolefins, in the presence of a molybdate catalyst to form more highly unsaturated aliphatic hydrocarbons.
Unsaturated aliphatic hydrocarbons, such as monoolefins and diolefins, are useful as monomers and comonomers in the preparation of polyolefin plastics.
U.S. Pat. No. 3,119,111 discloses a process for the oxidative dehydrogenation of a C.sub.4 to C.sub.6 alkane having a four carbon chain to a 1,3-alkadiene. The reaction occurs in the presence of oxygen and a catalyst containing an alkali metal molybdate, such as lithium molybdate. It is taught that the catalyst can be employed with a carrier material, such as powdered alumina. Disadvantageously, this process requires potentially explosive mixtures of alkanes and oxygen. More disadvantageously, the catalyst of this process contains a high concentration of alkali metal which lowers catalytic activity.
U.S. Pat. No. 3,180,903 discloses a process for the dehydrogenation of aliphatic hydrocarbons containing from two to five carbon atoms. Butanes, for example, can be converted to butenes and butadienes. The catalyst is taught to contain chromium oxides or molybdenum oxides supported on a gel-type alumina. Optionally, the catalyst may contain one or more alkali metal oxides. Disadvantageously, this process is limited to a low hydrocarbon conversion and a low ultimate yield of butadiene.
U.S. Pat. No. 3,488,402 teaches the dehydrogenation of butane to butene and butadiene in the presence of two catalysts. The first is a dehydrogenation catalyst containing alumina, magnesia, or combinations thereof, promoted with an oxide of a metal of Groups IVB, VB or VIB, such as chromia, vanadium oxide or molybdenum oxide. The second catalyst is an oxidation catalyst comprising a Group IVA or VA vanadate, molybdate, phosphomolybdate, tungstate or phosphotungstate. Disadvantageously, this process comprises two steps and requires subatmospheric pressures. Even more disadvantageously, this process produces low butadiene selectivity and yield.
U.S. Pat. No. 3,862,256 discloses a process for the oxidative dehydrogenation of paraffin hydrocarbons, such as butane, over a catalyst containing oxy compounds of molybdenum and magnesium, and optionally, vanadium and/or silicon. When butane is contacted with the catalyst, the products include butenes and butadiene; however, the selectivity and space-time yield of butadiene are lower than desired. In addition, the feed contains hydrocarbon and oxygen, which is not desirable for safety reasons. Finally, the magnesium oxide support does not possess the toughness and attrition resistance needed for fluid bed or transport reactors.
U.S. Pat. No. 4,229,604 discloses a process for the oxidative dehydrogenation of a paraffin, such as butane, to unsaturated hydrocarbons, such as butenes and butadiene. The catalyst is an oxide of molybdenum deposited on a carrier. The carrier is selected from the group consisting of granulated porous crystalline silica modified with magnesia, magnesium-titanium oxides, and magnesium-aluminum oxides. It is taught that during the carrier preparation silicates of the alkali metals or titanates or aluminates of magnesium are formed. It is further taught that on the surface of the catalyst there exists an active magnesium molybdate. Disadvantageously, the catalyst produces a selectivity and space-time yield of butadiene which are too low for industrial use. The low activity of this catalyst is attributed in part to its low surface area.
U.S. Pat. No. 4,388,223 discloses the oxidizing dehydrogenation of butene-1 to butadiene. The catalyst comprises (a) a crystalline phase (I) consisting of one or more molybdates belonging-to the monoclinic system, chosen from ferric, aluminum, cerium, and chromium molybdates, (b) a crystalline phase (II) consisting of one or more molybdates belonging to the monoclinic system, including magnesium molybdate, and (c) one or more promoter elements including vanadium. It is also taught that the catalyst may comprise alkaline elements such as potassium, lithium, cesium and magnesium and/or acidic elements, such as phosphorus and silicon. This process co-feeds hydrocarbon and oxygen, which is undesirable for safety reasons. Moreover, the catalyst does not have the toughness and attrition resistance required for fluid bed or transport reactors.
U.S. Pat. No. 3,769,238 teaches a catalyst composition comprising (a) a catalytically-active material containing a divalent metal, such as magnesium, tetravalent molybdenum, and oxygen in chemically combined form, and (b) a support comprising deacidified alumina. The support is deacidified with a small amount of a Group IA metal oxide, such as cesium oxide. The composition of the catalyst, stated in gram-atoms of metal per 100 moles of alumina, is as follows: divalent metal, 1 to 60 gram-atoms; molybdenum, 1.5 to 90 gram-atoms; Group IA metal, 1 to 5 gram-atoms.
While the oxidation of aliphatic hydrocarbons is well researched in the prior art, the selectivity and space-time yield to particular unsaturated hydrocarbons, such as diolefins, fall short of those which are desired for commercial exploitation. Moreover, the catalysts employed in the prior art do not possess the toughness and attrition resistance required for use in industrial fluid bed or transport reactors. Accordingly, it would be desirable to have a selective, direct oxidation of an aliphatic hydrocarbon, such as an alkane or monoolefin, to the corresponding unsaturated aliphatic hydrocarbons, specifically the diolefin. It would be more desirable if such an oxidation produced a high selectivity and high productivity of the diolefin and other olefins, and correspondingly low selectivities to deep oxidation products, such as carbon dioxide. Finally, it would be most desirable if the above-identified process could be accomplished with a catalyst of high attrition resistance so as to be useful in a commercial scale fluid bed or transport reactor.